1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of isomers of toluene diisocyanate. More specifically, the invention relates to a process for separating 2,6-toluene diisocyanate from the other toluene diisocyanate isomers by employing a solid bed adsorption system.
2. Background Information
The isomers, 2-4-toluene diisocyanate and 2,6-toluene diisocyanate are important starting materials for making polyurethanes which are useful in many applications as rigid or flexible forms or as fibers, e.g., insulation, soundproofing, interlinings for clothing and sleeping bags, cushions, spandex, etc.
It is common industrial practice to make polyurethane from a mixture of the isomers, 2,4- and 2,6-toluene diisocyanate (TDI), for example 80/20 or 65/35, derived from 2,4- and 2,6-toluene diisocyanate, because it is difficult and expensive to separate them by existing techniques. Current methods of separating the isomers involve crystallization and hence, are time-consuming. Moreover, polyurethanes synthesized from the pure 2,4- and 2,6-toluene diisocyanate have dramatically different properties compared to materials synthesized from their mixtures. I have found that prepolymers made with 2,6-toluene diisocyanate react about 5 to 10 times faster with a polyol than an 80/20 mixture of 2,4-toluene diisocyanate. Also, the 2,6-toluene diisocyanate system has been found to yield a significantly higher moduli than the 2,4-toluene diisocyanate system when the polyol is a polyester, e.g. butanediol adipate. Both tear resistance and tear propagation resistance are substantially higher for the 2,6-system when polyesters are used as the polyol. Accordingly, it is desirable to separate the TDI isomers by an economical process.
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. Furthermore, X and Y zeolites have been employed in a number of processes to separate individual hydrocarbon isomers.
It is known from U.S. Pat. No. 3,069,470 to Fleck et al, to use type X zeolites for the separation of the meta isomer from other isomers of toluidine. From U.S. Pat. No. 4,480,129, it is known that X and Y type zeolites, exchanged with transition metals, are para-selective in a mixture of isomers of toluidine.
U.S. Pat. No. 4,061,662 discloses the adsorption of unreacted toluene diisocyanate from polyisocyanate with X-type zeolites. U.S. Pat. No. 4,169,175 discloses removal of less than 0.7% unreacted toluene diisocyanate from urethane prepolymers with X-type zeolites.
Yabroff U.S. Pat. No. 4,246,187 discloses a method for separating the 2,4- and 2,6- isomers of toluene diisocyanate involving steps of crystallizing and centrifuging.
In U.S. Pat. No. 3,575,820, it is disclosed that ortho isomers of toluene diisocyanate can be removed from toluene diisocyanate mixtures by incorporating aluminum oxide which will polymerize the ortho isomers, whereupon the non-vicinal isomers can be separated by distillation. Chemical Abstract 101:116099X (1984) discloses a treatment for removing toluene diisocyanate from waste gas by adsorption with activated carbon.
In Japanese Patent Application No. 56905/79, publicly disclosed on Nov. 20, 1980, it is disclosed that a solid adsorbent containing titanium oxide will selectively adsorb the para-isomer of toluidine.
It is known from U.S. Pat. No. 4,270,013 to Priegnitz et al that ortho-nitrotoluene may be separated from other nitrotoluene isomers by using a type-X zeolite containing at exchangeable cationic sites one cation selected from a group that includes potassium and barium. The specific desorbent materials disclosed by this reference are toluene and 1-hexanol. The separation of isomers of di-substituted benzenes with crystalline aluminosilicates having silica/alumina mole ratio of at least 12 is disclosed in my U.S. Pat. No. 4,467,126.